The present invention generally relates to an optical scanning apparatus, and particularly to a post-objective type optical scanning apparatus. The present invention more particularly relates to a post-objective type optical scanning apparatus which employs a polygon lens which functions as a rotary deflection mechanism.
Currently, an electrophotography printer and a hard copy machine are known. The electrophotography printer records an image on general paper, which is not specific paper such as developing paper. An optical printer, a digital copier and a facsimile machine are examples of the electrophotography printer. The hard copy machine records an electrically processed image on a film, developing paper or the like. These machines will be further developed to become more compact and economic. To attain such machines, it is necessary to reduce an optical scanning apparatus, or optical scanner in size. In the optical scanner, a modulated light beam is scanned directly on a storage medium or on an intermediate medium, so that an image is formed thereon.
Conventionally, most of optical scanners utilize a polygon mirror. An example of those is described with reference to FIG. 1. Referring to FIG. 1, a light source 1 is made up of a light-emitting part 2 constituted by a laser diode, and a collimator lens 3. A parallel light beam is extracted from the collimator lens 3. The parallel light beam passes through a cylindrical lens 4, and then enters an optical scanner 7. The optical scanner 7 includes a polygon mirror 6, which is driven by a scanner motor 5 so as to rotate in a direction of an arrow at a constant rotation speed. An incident light beam from the cylindrical lens 4 is reflected on the polygon mirror 6, and is changed to a scanning light beam represented by a solid line. The scanning light beam is refracted by an F.theta. lens 8, and then passes through a longitudinal cylindrical lens 9. Then the scanning light beam scans a photosensitive drum 10 in a direction of an arrow of a broken line. This direction is a main scanning direction. A broken line 11 denotes a scanning line on a peripheral surface of the photosensitive drum 10. A scanned portion of the photosensitive drum 10 is exposed. The photosentitive drum 10 is rotated by a main motor (not shown) in a direction of an arrow of a solid line. This direction is a sub-scanning direction. In this manner, an image is formed on the peripheral surface of the photosensitive drum 10.
The above-mentioned optical scanner employs a pre-object type optical system, in which the f.theta. lens 8 (an image-formation lens) is interposed between the optical scanner 7 and the photosensitive drum 10. The optical scanner 7 of FIG. 1 has an advantage that a scanning speed can be optically compensated by using the f.theta. lens 8. On the other hand, an f.theta. lens of a large size must be used, because the scanning light beam from the f.theta. lens 8 must scan the entire peripheral surface of the photosensitive drum 10 in the direction of the arrow. For this reason, it is difficult to fabricate a compact and economic optical scanner.
To reduce the above disadvantages, there has been proposed a post-objective type optical scanner (see the Japanese Laid-Open Patent Application Nos. 61-156020 and 61-242459). The disclosed post-objective type optical scanner employs, in place of the f.theta. lens, an image-formation lens positioned between a light source and an optical scanner. A conventional post-objective type optical scanner is shown in FIG. 2. In FIG. 2, those parts which are the same as those parts in FIG. 1, are given the same reference numerals.
Referring to FIG. 2, an image-formation lens 12 is positioned between the light source 1 and the optical scanner 7. The f.theta. lens 8 shown in FIG. 1 is removed. The image-formation lens 12 functions to form a spot of the parallel light beam on the peripheral surface of the photosensitive drum 10. The image-formation lens 12 may be constituted by an image formation lens having a smaller diameter, because the image-formation lens 12 handles the parallel beam which does not spread out widely. With the arrangement of FIG. 2, a compact and economic optical scanner may be fabricated.
However, the conventional post-objective type optical scanner has disadvantages described below. In the arrangement of FIG. 2, the focal length of the image-formation lens 12 is constant, and a distance between the image-formation lens 12 and a reflection point on the polygon mirror 6 is constant. Therefore, as shown in FIG. 3 (wherein the longitudinal cylindrical lens 9 is omitted for the sake of simplicity), a locus of the beam spot formed by the rotation of the polygon mirror 6 forms an arc. A center of the locus in the form of the arc is the reflection point 13 of the polygon mirror 6. The arc-shaped locus has a radius which corresponds to a distance between the reflection point 13 and the photosensitive drum 10. Therefore, curvature of an image-formation surface S arises. The curvature of the image-formation surface is not great in a vicinity of a center C of the projection plane which is the peripheral surface of the photosensitive drum 10. However, the larger an angle of deflection .theta., the longer a distance m between the projection plane and the image-formation surface S. As a result, the spot of the scanning light beam on the projection plane becomes blurred.